Method of de-oxidizing metal surfaces



United States Patent 2,878,189 METHOD OF DE-OXIDIZING METAL SURFACES Anthony J. Certa, Bridgeport, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application December 16, 1954 Serial No. 475,848

4 Claims. (Cl. 252-146) The present invention relates to a novel method for removing oxide from oxidized surfaces of metal articles made from certain alloys; and, more particularly, the invention relates to a novel method for removing oxide from nickel-cobalt-iron alloy articles surfaces of which contain an oxide coating.

There is presently available a group of alloys consisting principally of nickel, cobalt and iron which are particularly suitable for making small metal articles and parts. Because of the expansion characteristics and other properties of these alloys, glass can readily be sealed thereto. Such alloys, therefore, find especial use in the manufacture of electrical and electronic parts.

In order to fuse glass to these alloys, the surface of the metal article must be heavily oxidized since the molten glass readily wets the oxide surface but not the metal itself. This oxidizing procedure generally involves heating the metal article to a temperature on the order of 750 C. in an atmosphere of water vapor. After the bonding operation, the exposed metal surfaces of the composite glass-metal article still contain a heavy oxide layer.

In many applications, such articles must be resistant to corrosion but the alloys are not as corrosion resistant as is required in many applications. It therefore becomes necessary to provide a corrosion-resistant coating on the surface of the metal parts. However, because of the stated heavy oxide layer, it is impossible to deposit an adherent layer of corrosion-resistant metal directly thereover,"and the oxide must first be removed. In view of the'very critical dimension tolerances at times required in such fine metal parts, the oxide layer must be removed without any attack on the underlying metal.

In removing oxide films from metal surfaces, one normally thinks of the use of an acid. However, tests with the common acids show that, as such, they are not commercially suitable. Thus phosphoric acid, hydrochloric acid and hot nitric acid rapidly attack the underlying metal. Cold nitric acid will not readily remove the oxide. Cold concentrated sulfuric acid does not have any significant effect on the oxide layer. Hot concentrated sulfuric acid rapidly removes the oxide, and immediately attacks the base metal.

It is the principal object of the present invention to provide a method for removing oxide from articles made from cobalt-nickel-iron alloys without danger of attacking the underlying metal.

It is another object of the present invention to provide a novel chemical method for removing oxide from articles made from cobalt-nickel-iron alloys wherein the oxide layer may be rapidly removed without attacking the underlying metal exposed to the bath upon removal of the oxide layer.

2,818,189 Patented Mar. 17, 1959 Other objects, including the provision of a novel bath for removing the oxide, will become apparent from a consideration of the following specification and the claims.

The method of the present invention comprises immersing the article of cobalt-nickel-iron alloy having an oxide film thereon in a bath consisting essentially of an aqueous solution of sulfuric acid and ammonium sulfate, at an elevated temperature, until the surface of the metal is clean.

The treated article may then be removed from the bath, and the time of removal is not overly critical once the oxide has been removed. Thus, the article may be immersed in the bath and left there without careful inspection, and, once the oxide is removed, the article may be left in the bath for a reasonable period of time without any attack on the metal itself. In this connection, treated articles have been left in the bath maintained at about 95-100" C. for as long as three-quarters of an hour without any apparent attack on the metal. At lower temperatures the articles may be left in the bath for increasingly long periods of time without attack on the base metal.

The method of the present invention is, as stated, applicable to the treatment of articles prepared from nickelcobalt-iron alloys, which have an oxide on the surface thereof. Since such alloys are highly suitable for heat sealing to glass (which, as stated, requires that the surface to which the glass is sealed be highly oxidized before the sealing operation) the present method is par ticularly applicable to the treatment of articles comprising a glass member or members sealed to a member or members prepared of the stated alloy, the exposed surfaces of which metal member or members still retain the oxide layer. Such articles are often used in the electrical or electronic field, and, because of their generally small size and critical dimensions, it is one of the important features of the present invention that the oxide can be removed without in any way altering the dimensions of the metal part or parts.

As stated, the metal articles treated in accordance with the present invention are nickel, cobalt, iron alloys. Such alloys generally contain between about 24% and about 34%, by weight, of nickel, between about 5 and about 25%, by weight, of cobalt, the balance being substantially iron. Small amounts of other metals, such as manganese, may also be present in amounts below about 1%. An especially advantageous alloy in this connection is that containing between about 28% and about 30% of nickel, between about 16% and about 18% of cobalt and between about 52% and about 54% of iron, sold under the trade name of Kovar by the Stupakoff Ceramic and Manufacturing Company.

The bath employed in accordance with the present method consists principally of sulfuric acid, ammonium sulfate and water. As between the sulfuric acid and water, the concentration of sulfuric acid (as 66 B. H in the water during at least the major part of the process is advantageously between about 15 and about 30%, by volume. This may be readily provided by mixing concentrated sulfuric acid (-98%, 66 B.) with water in a ratio of about one part of the former to between about two and a half and about five parts of the latter. Preferably, the concentration of the sulfuric acid in the water is between about 20 and about 25%, by volume. Actually, in preparing the solution initially, the concentration of the acid may be below the above ranges,

3 evaporation of water in heating up the solution for use being relied upon to raise the concentration of acid therein to the desired point.

With respect to the ammonium sulfate in the bath, it

has been found that the concentration thereof should be relatively high. in order to avoid significant attack on the underlying metal, it has been found that the concentration of the ammonium sulfate in the bath should be at least about 75 grams thereof per 100 ml. of the aqueeous sulfuric acid solution. Best results are obtained When the concentration is at least about 100 grams of ammonium sulfate per 100 ml. of solution. The concen tration of ammonium sulfate in the bath may, and preferably does, go as high as the saturation point for that salt in the bath at the temperatures of operation discussed more in detail hereinafter. At a temperature of about 100 C., the acid solution is saturated with the ammonium sulfate at a concentration of about 125 grams of ammonium sulfate per 100 ml. of acid solution. Thus, in accordance with the particularly preferred embodiment of the present process, the acid bath, at the temperature of operation, will be substantially saturated with the ammonium sulfate. Of course, excess ammonium sulfate may be employed, the excess over and above that providing a saturated solution, merely existing as such in solid form in the bath.

7 It will be seen that the bath employed in accordance with the present invention consists essentially of sulfuric acid in water which solution contains a relatively large amount of ammonium sulfate dissolved therein. It has been found that the bath should be substantially free of strong acid anions other than sulfuric acid. Thus, no other strong acid or strong acid salt is relied upon to remove the oxide, and substantial amounts of such acid or salt and of even weak acids and salts of weak acids (the exact amount depending upon the particular acid or salt) in addition to the sulfuric acid and ammonium sulfate in the stated amounts may have a deleterious efiect on the base metal. Accordingly, ammonium salts other than ammonium sulfate, such as ammonium nitrate, ammonium chloride, ammonium phosphate, ammonium salts of organic acids, and the like, are not relied upon to provide the ammonium sulfate in solution. Of course, an ammonium salt having a volatile anion, such as ammonium carbonate, may be employed to provide some or all of the ammonium sulfate in solution, since, at the temperature of operation, carbondioxide will be evolved and released from the bath. Ammonium hydroxide may also be employed. However, in these cases, consideration must be had of the amount of sulfuric acid required tojcombine with the ammonium ions to provide ammonium sulfate in solution. Preferably, of course, ammonium sulfate itself is added to and dissolved in the bath. While, as stated, the principal oxide-removing ingredients of the bath of the present invention are the sulfuric acid and ammonium sulfate in water, it will be understood that minor amounts of other materials not substantially deleteriously affecting the characteristics of the bath may be employed to impart or augment certain. features. For example, metals such as copper, nickel, iron, cobalt, or the like, may be included in the bath to serve as scavengers, activators or inhibitors.

In accordance with the process of the present invention, the bath is at an elevated temperature at the time the oxide is removed from the alloy articles. At temperatures below about 80 C., the time required for removal of the oxide may be too long for practical application, although, if time is not an important consideration, temperatures as low as 50-60 C. may be employed. At temperatures of about 80 C. and above the oxide will be'removed within a reasonable time, and the higher the temperature the faster the removal of the oxide. The temperature of thebath ma'ygo up to the boiling point of thesolution which is in the neighborhood of about 102-108 0., depending upon the exact concenttati on" of sulfuric acid and ammonium sulfate in the bath. Higher temperatures can be employed by the use of super atmo's pheric pressures, however, no significant advantage is to be gained by operating at such temperatures. Preferably, the bath is maintained slightly below the boiling point, especially within the range of between about and about 100 C.

The time required for removal of the oxide will vary somewhat depending upon the thickness of the oxide film and upon the exact conditions employed during the process. In general, the oxide will be substantially completely removed within about 3 to about 8 minutes after immersion in the bath at operating temperature. In the event the article has a slight greasy or oily film thereon from, for example, handling, somewhat longer times may be required.

After removal of the oxide in accordance with the present process, the article may be removed from the bath and washed, such as with water, to remove adhering bath solution. After washing, the article is preferably dried. After the article has been removed from the bath, washed and dried, it is in proper condition for further treatment, such as vapor-, chemicalor electroplating. The process of the present invention will be more readily understood from a consideration of the following specific example which is given for the purpose of illustration only and is not intended to limit the scope of the invention in any way.

Example Hollow diode pins made of an alloy having the following composition: nickel 29%, cobalt 17%, manganese 0.3% and iron the balance (Kovar of Stupakoif Ce ramic and Manufacturing Company) are oxidized at 750 C. The ends of two such pins are sealed to each end of a small glass tube by fusion of the glass. The exposed surfaces of the diode pins are covered with a heavy black layer of oxide.

A bath is prepared by adding 20 ml. of commercial concentrated sulfuric acid (-98% H 80 to 80 ml. of water, with stirring. The heat generated by this addition raises the temperature of the resulting solution to about 50-60 C. 125 grams of ammonium sulfate are then added and the mixture heated with stirring to about 95 C. At this point substantially all of the ammonium sulfate has gone into the solution.

While maintaining the bath at about 95 C., the diode assemblies are immersed in the bath and left therein for about ten minutes. The assemblies are then removed, washed in water, and dried. The exposed metal parts of the assemblies have a clean, fairly bright surface, ideally suited for electroplating. The diode assemblies may be left in the bath for as long as three-quarters of an hour at the operating temperature without any apparent attack on the base metal.

Modification is possible in the selection of ingredients of the bath as well as in the proportion thereof, and in the exact technique followed in carrying out the process, without departing from the scope of the invention.

I claim:

1. The method of removing oxide from the surface of an oxidized cobalt-nickel-i-ron alloy article without attack on the metal itself which comprises immersing the oxidized article in a bath, at a temperature above about 50 C., consisting essentially of ammonium sulfate in aqueous sulfuric acid, at least until oxide has been removed from said surface, the concentration of sulfuric acid inthe water being between about 15 and about 30%, by volume, calculated as 66 B. H 80 and said monium sulfate being present in an amount of at least about 75 grams per ml. of aqueous sulfuric acid. N

Z. The method of claim 1 wherein the temperature of the bath is between about 80 C. and the boiling point;

and wherein said ammonium sulfate is present in an amount of at least about 100 grams thereof per 100 ml. of aqueous sulfuric acid.

3. The method of claim 1, wherein said alloy contains between about 24 and about 34% of nickel, between about 5 and about 25% of cobalt and the balance substantially iron.

4. The method of claim 2 wherein the temperature of the bath is between about 90 and about 100 C.; and wherein said aqueous sulfuric acid is substantially saturated with said ammonium sulfate.

References Cited in the file of this patent UNITED STATES PATENTS Gravell Mar. 17, 1931 Urban Jan. 23, 1945 Shawcross Apr. 15, 1947 Snyder May 15, 1951 De Holt Ian. 12, 1954 Brundin Nov. 16, 1954 FOREIGN PATENTS Great Britain Sept. 9, 1948 

1. THE METHOD OF REMOVING OXIDE FROM THE SURFACE OF AN OXIDIZED COBALT-NICKEL-IRON ALLOY ARTICLE WITHOUT ATTACK ON THE METAL ITSELF WHICH COMPRISES IMMERSING THE OXIDIZED ARTICLE IN A BATH, AT A TEMPERARURE ABOVE ABOUT 50*C., CONSISTING ESSENTIALLY OF AMMONIUM SUFLATE IN AQUEOUS SULFURIC ACID, AT LEAST UNTIL OXIDE HAS BEEN REMOVED FROM SAID SURFACE, THE CONCENTRATION OF SULFURIC ACID IN THE WATER BEING BETWEEN ABOUT 15 AND ABOUT 30% BY VOLUME, CALCULATED AS 66* BE''. H2SO4, AND SAID AMMONIUM SULFATE BEING PRESENT IN AN AMOUNT OF AT LEAST ABOUT 75 GRAMS PER 100 ML. OF AQUEOUS SULFURIC ACID. 